Sunday, November 16, 2014

Concrete – Part 3 – Understanding the basics - Chloride Attack, Alkaline Silica Reaction, Sulphate Attack & Others

In last week’s article I considered carbonation and in the final part of this ‘mini-series’ I will focus on other concrete defects, such as Chloride Attack, Alkaline Silica Reaction and Sulphate Attack amongst other concrete defects

Over the last few weeks I have discussed the wide use of concrete as a construction material, considered its many positive attributes and also explained the vulnerability of concrete to certain defects.  In last week’s article I considered carbonation (Link) and in this final part of this ‘mini-series’ I will focus on other concrete defects.

Chloride Attack – Chloride finds its way into and through concrete in a similar process to carbonation, due to its porous nature. Chloride ions which are introduced into concrete from de-icing salts or are within or in close proximity to marine environments can attack concrete aggressively resulting in a faster rate of deterioration compared to carbonation.  When chloride passes through the concrete and eventually reaches any reinforcement, corrosion will occur. Salt is a mineral substance which consists primarily of sodium chloride.  When the sodium chloride is dissolved in water, which may be present in the pores of concrete, a versatile, highly corrosive and mobile solution is formed of sodium ions (Na+) and chloride ions (Cl-).  Once this solution comes into contact with any reinforcement it will attach the passive layer which protects it. The reinforcement will then corrode in the presence of air and water, resulting in corrosion.  This will result in cracking and spalling which will appear very similar to the effects of carbonation.

The consequence of chloride attack can be seen on the underside of road bridges and buildings and structures in close proximity to the coast. As discussed above, the impact of chloride attack will appear very similar to carbonation, so it will be necessary to not only consider the environment but also undertake testing to confirm the cause.

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Alkaline Silica Reaction – A good explanation of alkaline silica reaction is defined by as ‘Alkali-silica reaction takes place between reactive siliceous minerals in certain aggregates and OH- (hydroxide ions) in the cement paste. Alkalis (Na+ and K+) from the cement, mixing water, or environment increase the concentration of OH- ions in the concrete. The OH- ions attack susceptible aggregate minerals. The damaged framework forms a gel that absorbs water from the surrounding concrete. The gel expands, generating pressures that can crack the concrete. The damage may not be visible to the naked eye for years after the concrete has been placed’

Alkaline silica reaction can usually be identified by random cracking on the surface of the concrete and in advanced cases, a gel like substance may be visible or possible spalling of the concrete. Cracking usually appears in areas with a regular supply of moisture, such as close to the watercourses and ground behind retaining walls etc. In order to confirm the presence of alkaline silica reaction it is necessary for core samples of the concrete to be taken and put under a microscope to establish their mineralogical and chemical characteristics, this is something referred to as petrographic testing.

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Sulphate Attack – Again, the porous nature of concrete makes it vulnerable to sulphate attack in the same manner as previously discussed for other defects.  Water containing dissolved sulphates such as sodium sulphate, potassium sulphate or magnesium sulphate can penetrate into the concrete and as it progresses, the composition and microstructure of the concrete will be changed.  This can then result in cracking, expansion and loss of bond between the cement paste and the aggregate, which often results in a loss of strength of the concrete. 

Possible sources of sulphates include seawater, oxidation of sulphide minerals in clay (such as copper) adjacent to the concrete (this can produce sulphuric acid which reacts with the concrete), bacterial action in sewers (anaerobic bacteria produces sulphur dioxide which dissolves in water and then oxidizes to form sulphuric acid), In masonry, sulphates are present in bricks and can be released over a long period of time, causing sulphate attack of mortar.

In the UK sulphate attack is particularly common in ‘older’ solid concrete ground floors.  The use of fill material became very popular for residential solid concrete ground floor construction from the early 1940s. In the early post war years, waste materials such as burnt colliery shale, blast furnace slag and red ash were promoted by the government as appropriate materials to use for this purpose. However, it was later discovered that these types of fill materials contained high levels of sulphates which often resulted in significant problems and associated expensive remedial works to replace affected floors. Sulphates from these fill materials, with the presence of water, would attack the tri-calcium aluminate (one of the components of Portland Cement), within the concrete, which would result in lifting, expansion and cracking of the concrete floor slab.  This problem was significantly reduced with the use of appropriate fill that did not contain these high levels of sulphates, together with the introduction of damp proof membranes (to reduce water penetration) and insulation to improve thermal efficiency. Replacement of a solid ground floor with a new floor is the only practical way of dealing with sulphate attack, which as you can see from the image below can be very disruptive as well as expensive.

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Other concrete defects – There are a number of other concrete defects that may be identified including ‘honeycombing’ concrete which occurs where wet concrete has not had all of the air taken out of it due to poor workmanship and lack of quality control.  In order to remove any air within wet concrete a vibrating poker is put into the wet concrete which agitates the wet mix and ensures that air voids are removed before the concrete cures (hardens).  If care is not taken during the installation process, or the installation is rushed, voids or honeycombs will be visible in the concrete when the formwork is struck (removed), similar to that identified in the image below.

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Plastic Shrinkage – small cracks appear in the surface of freshly laid wet concrete soon after it has been placed, while it is still wet or plastic. Plastic shrinkage cracking is highly likely to occur when high evaporation rates cause the concrete surface to dry out too quickly, before it has the opportunity to set.  This is particularly problematic when wet concrete is being laid in high temperatures and highly humidity.  In order to slow down or control the curing process, damp or wet hessian may be laid over the newly laid concrete which is repeatedly wet to allow the water within the concrete to evaporate out at a much more natural rate, which will significantly reduce the risk of plastic shrinkage occurring.

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Over the last few weeks I have discussed some of the common defects that occur in concrete however you may come across other issues relating to expansion joints or insufficient cover, which primarily relate to poor design or poor workmanship.  These articles can become very technical and sometimes quite complex, hopefully the information provided has introduced some of the defects and may motivate you to undertake more in depth research for some of the defects considered.

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